Find the Celebrity

Source

• leetcode: 277

Suppose you are at a party with n people (labeled from 0 to n - 1) and
among them, there may exist one celebrity. The definition of a
celebrity is that all the other n - 1 people know him/her but he/she does
not know any of them.

Now you want to find out who the celebrity is or verify that there is
not one. The only thing you are allowed to do is to ask questions
like: "Hi, A. Do you know B?" to get information of whether A knows B. You
need to find out the celebrity (or verify there is not one) by asking as
few questions as possible (in the asymptotic sense).


You are given a helper function bool knows(a, b) which tells you whether A

knows B. Implement a function int findCelebrity(n), your function should
minimize the number of calls to knows.

Note: There will be exactly one
celebrity if he/she is in the party. Return the celebrity's label if there
is a celebrity in the party. If there is no celebrity, return -1.

Java

int findCelebrity(int n) {
    if(n<=1) return n;
    int candidate = 0;

    for(int i=1; i<n; i++){

        if ( !knows(i,candidate) ){
            candidate = i;
        }
    }

    for(int j=0; j<n; j++){

        if(j== candidate) continue;

        if( !knows(j,candidate) || knows(candidate,j) ){
            //if j does not know candidate, or candidate knows j, return -1;
            return -1;
        }
    }
    return candidate;
}